Simultaneous uplink transmission in a wireless network

ABSTRACT

A method for use in a communication network includes generating a synchronization data unit to be transmitted to a plurality of devices, where the synchronization data unit specifies a space-time mapping parameter, and receiving a plurality of data units via a plurality of antennas, the plurality of data units are transmitted simultaneously from respective ones of the plurality of devices in accordance with the space-time mapping parameter.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the following U.S. ProvisionalPatent Applications:

U.S. Provisional Patent Application No. 61/227,356, entitled “UplinkMultiuser MIMO in WLAN,” filed on Jul. 21, 2009;

U.S. Provisional Patent Application No. 61/228,080, entitled “UplinkMultiuser MIMO in WLAN,” filed on Jul. 23, 2009;

U.S. Provisional Patent Application No. 61/230,619, entitled “UplinkMultiuser MIMO in WLAN,” filed on Jul. 31, 2009; and

U.S. Provisional Patent Application No. 61/254,256, entitled “UplinkMultiuser MIMO in WLAN,” filed on Oct. 23, 2009.

All of the above-referenced applications are hereby incorporated byreference herein in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and,more particularly, to multiple-input, multiple-output (MIMO) wirelesslocal area networks.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Wireless local area network (WLAN) technology has evolved rapidly overthe past decade. Development of WLAN standards such as the Institute forElectrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g,and 802.11n Standards has improved single-user peak data throughput. Forexample, the IEEE 802.11b Standard specifies a single-user peakthroughput of 11 megabits per second (Mbps), the IEEE 802.11a and802.11g Standards specify a single-user peak throughput of 54 Mbps, andthe IEEE 802.11n Standard specifies a single-user peak throughput of 600Mbps.

WLANs typically operate in either a unicast mode or a multicast mode. Inthe unicast mode, an access point (AP) transmits information to oneclient station at a time. In the multicast mode, the same information istransmitted to a group of client stations concurrently. In the uplinkdirection, a client station typically contends for access to the mediumor transmits data to the AP within a time period specifically allocatedto the client station.

SUMMARY

In an embodiment, a method for use in a communication network includesgenerating a synchronization data unit to be transmitted to a pluralityof devices, where the synchronization data unit specifies a space-timemapping parameter, and receiving a plurality of data units via aplurality of antennas, the plurality of data units are transmittedsimultaneously from respective ones of the plurality of devices inaccordance with the space-time mapping parameter.

In another embodiment, an apparatus includes a synchronizationcontroller to generate a synchronization data unit, where thesynchronization data unit is transmitted to a plurality of devices andspecifies a space-time mapping parameter, and a communication frameprocessor to process a signal received via a plurality of antennas,where the received signal includes a plurality of communication framestransmitted simultaneously from respective ones of the plurality ofdevices in accordance with the space-time mapping parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a block diagram of an example wireless local area network (WLAN)that utilizes access control techniques and/or physical layer (PHY)preamble formatting techniques in accordance with an embodiment of thepresent disclosure;

FIG. 2A is a diagram of a prior art data unit format used in acommunication system in which all devices support only a single spatialstream;

FIG. 2B is a diagram of a prior art data unit format used in acommunication system in which some devices support only a single spatialstream, and other devices support two spatial streams;

FIG. 2C is a diagram of a prior art data unit format used in acommunication system in which all devices support two spatial streams;

FIG. 2D is a diagram of a prior art data unit format used in acommunication system in which all devices support four spatial streams;

FIG. 2E is a diagram of a prior art sounding data unit format used in acommunication system in which some devices support only one spatialstream, and some devices support two spatial streams;

FIG. 2F is a diagram of a prior art sounding data unit format used in acommunication system in which all devices support two spatial streams;

FIG. 3 is a diagram illustrating communications in a communicationnetwork during carrier sense multiple access (CSMA) time periods and asimultaneous uplink transmission (SUT) time period, according to anembodiment;

FIG. 4A is a timing diagram illustrating simultaneous uplinktransmission during an SUT period followed by individualacknowledgements, according to an embodiment;

FIG. 4B is a timing diagram illustrating simultaneous uplinktransmission during an SUT period followed by a block acknowledgement,according to an embodiment;

FIG. 4C is a timing diagram illustrating simultaneous uplinktransmission of communication frames having non-equal length and anappropriate padding field, according to an embodiment;

FIG. 5A is a diagram of an example format of an SUT communication framethat includes a legacy portion and a very high throughput (VHT) portion,according to an embodiment;

FIG. 5B is a diagram of an example format of an SUT communication framethat includes only a VHT portion, according to an embodiment;

FIG. 5C is a diagram of another example format of an SUT communicationframe that includes a legacy portion and a very high throughput (VHT)portion, according to an embodiment;

FIG. 6A is a diagram of a portion of a PHY preamble that includesseveral long training fields for simultaneous transmission to an accesspoint (AP), according to an embodiment;

FIG. 6B is a diagram of a portion of a PHY preamble that includesseveral long training fields for staggered transmission to an AP,according to an embodiment;

FIG. 6C is a diagram of a portion of a PHY preamble that includesseveral long training fields for mixed simultaneous/staggeredtransmission to an AP, according to an embodiment;

FIG. 7 is a diagram of a portion of a communication frame in which a VHTsignaling portion is transmitted using a single space-time stream, and adata portion is transmitted using multiple space-time streams, accordingto an embodiment;

FIG. 8 is a diagram of a portion of a communication frame in which a VHTsignaling portion and a data portion are transmitted using multiplespace-time streams, according to an embodiment;

FIG. 9 is a block diagram of a communication frame generator that isused in an SUT-capable station, according to an embodiment;

FIG. 10 is a block diagram of a communication frame processor that isused in an SUT-capable AP, according to an embodiment; and

FIG. 11 is a flow diagram of an example method for scheduling an SUTtime period and communicating within the scheduled SUT time period inaccordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

In embodiments described below, several network devices simultaneouslytransmit independent data to another network device during a certaintime period. In an embodiment, the receiving device is an access point(AP) of a wireless communication network that receives simultaneousuplink transmission (SUT) signals such as communication frames (orsimply “frames”) from several client stations (or simply “stations”) viaan antenna array. To this end, in accordance with an embodiment, the APschedules a “protected” time period for use by SUT-capable stationsonly, provides synchronization data to SUT-capable clients, receives inparallel several communication frames of the same or different duration,and acknowledges the receipt of the communication frames in a singleacknowledgement frame or several station-specific acknowledgementframes. As discussed below, the AP in some embodiments also controls oneor more of (i) the power at which stations transmit SUT frames duringthe protected time period, (ii) the maximum duration of an SUT frame,(iii) the amount of bandwidth allocated to each station, etc., and/orone or more of (iv) assigns unique indexes to stations, (v) generatesmodulation and coding scheme (MCS) parameters for each station, (vi)allocates spatial (or “space-time”) streams to stations, etc. As alsodiscussed below, stations format the physical layer (PHY) preambles ofSUT frames so as to enable the AP and/or the stations to properlyestimate the MIMO channel between the receive (Rx) antenna array of theAP and a virtual transmit (Tx) antenna array formed by the antennas ofthe stations, in accordance to an embodiment.

Using the techniques for scheduling an SUT period and/or formatting aPHY preamble, network devices ensure that uplink data transmittedsimultaneously is properly synchronized, that power in uplinktransmissions is properly controlled (e.g., so that an excessivelystrong signal from one station does not “drown out” signals from otherstations), that SUT frames are properly acknowledged, etc. An examplecommunication network in which some or all of these techniques can beimplemented is described next, followed by a discussion of several priorart formats of a PHY preamble and a corresponding mathematical model,then followed by a discussion of several example embodiments of anaccess protocol according to which an AP and/or client stations schedulean SUT time period according to an embodiment, and further followed by adiscussion of several example formats of a PHY preamble that the AP andthe client stations can utilize.

FIG. 1 is a block diagram of an example wireless local area network(WLAN) 10, according to an embodiment. An AP 14 includes a hostprocessor 15 coupled to a network interface 16. The network interface 16includes a medium access control (MAC) unit 18 and a physical layer(PHY) unit 20. The PHY unit 20 includes a plurality of transceivers 21,and the transceivers are coupled to a plurality of antennas 24. Althoughthree transceivers 21 and three antennas 24 are illustrated in FIG. 1,the AP 14 can include different numbers (e.g., 2, 4, 5, etc.) oftransceivers 21 and antennas 24 in other embodiments.

The WLAN 10 includes a plurality of client stations 25. Although fourclient stations 25 are illustrated in FIG. 1, the WLAN 10 can includedifferent numbers (e.g., 2, 3, 5, 6, etc.) of client stations 25 invarious scenarios and embodiments. Two or more of the client stations 25are configured to transmit corresponding data streams to the AP 14 sothat the AP 14 receives the data streams simultaneously.

A client station 25-1 includes a host processor 26 coupled to a networkinterface 27. The network interface 27 includes a MAC unit 28 and a PHYunit 29. The PHY unit 29 includes a plurality of transceivers 30, andthe transceivers are coupled to a plurality of antennas 34. Althoughthree transceivers 30 and three antennas 34 are illustrated in FIG. 1,the client station 25-1 can include different number (e.g., 1, 2, 4, 5,etc.) of transceivers 30 and antennas 34 in other embodiments.

The client stations 25-2 and 25-3 have a structure that is the same asor generally similar to the client station 25-1. In an embodiment, eachof the client stations 25-2 and 25-3 is structured like the clientstation 25-1 but has only two transceivers and two antennas. Together,the three antennas 34 of the client station 25-1, the two antennas ofthe client station 25-2 and the two antennas of the client station 25-3form a virtual antenna array 35. In an embodiment, the AP 14communicates data over a MIMO channel defined, at the one end, by thearray including the antennas 24-1, 24-2, and 24-3 and, at the other end,by the virtual antenna array 35. The client stations 25-1, 25-2, and25-3, as well as similar stations capable of transmitting independentuplink data to the AP 14 or a similar network device, are referred toherein as “SUT-enabled stations” or “SUT-capable stations.”

In an embodiment, each of the client stations 25-1, 25-2, and 25-3 isconfigured to transmit multiple spatial streams via multiple transmitantennas. When space-time coding is employed, the multiple spatialstreams are sometimes referred to by those of ordinary skill in the artas space-time streams. If the number of space-time streams is less thanthe number of transmit chains, spatial mapping and/or beamforming isemployed, in some embodiments.

According to an embodiment, the legacy client station 25-4 is a legacyclient station that is not enabled to transmit a data stream to the AP24 at the same time that other client stations 25 transmit data to theAP 14. Further, in accordance with an embodiment, the legacy clientstation 25-4 includes a PHY unit that is generally capable oftransmitting a data stream to the AP 14 at the same time that otherclient stations 25 transmit data to the AP 14, but the MAC unit of thelegacy client station 25-4 is not enabled with MAC layer functions thatsupport transmitting a data stream to the AP 14 at the same time thatother client stations 25 transmit data to the AP 14. The structure ofthe client station 25-4 is generally similar to the structure of theclient station 25-1, according to an embodiment.

In an embodiment, the legacy client station 25-4 operates according tothe IEEE 802.11a and/or the IEEE 802.11n Standards. During operation,the SUT-capable stations and the legacy client station 25-4 utilize thecommunication channel as described in the in U.S. patent applicationSer. No. 12/758,603, filed on Apr. 12, 2010 and entitled “Physical LayerFrame Format for WLAN,” (“the '603 application”), the entire disclosureof which is expressly incorporated by reference herein. For example, inaccordance with the techniques described in the above-referencedapplication, the legacy client station 25-4 occupies an entirecommunication channel in accordance with the IEEE 802.11a Standard whencommunicating with the AP 14, so that the AP 14 transmits data to thelegacy client station 25-4 in 64 orthogonal frequency divisionmultiplexing (OFDM) subchannels, and the legacy client station 25-4transmits data to the AP 14 in the 64 OFDM subchannels. As anotherexample, when the AP 14 and the legacy client station 25-4 communicateaccording to the IEEE 802.11n Standard using a 40 MHz channel, the AP 14transmits data to the legacy client station 25-4 in 128 OFDM subchannelsthat occupy the entire channel, and the legacy client station 25-4transmits data to the AP 14 in the 128 OFDM subchannels.

On the other hand, the AP 14 is configured, in accordance with anembodiment, to partition the wider communication channel into narrowersub-bands or OFDM sub-channel blocks, and different data streams fordifferent client devices 25 are transmitted in respective OFDMsub-channel blocks, in accordance with the techniques described in the'603 application. According to an embodiment, each OFDM sub-channelblock substantially conforms to the PHY specification of the IEEE802.11a Standard. According to another embodiment, each OFDM sub-channelblock substantially conforms to the PHY specification of the IEEE802.11n Standard. According to another embodiment, each OFDM sub-channelblock substantially conforms to a PHY specification of a communicationprotocol other than the IEEE 802.11a and the IEEE 802.11n Standards. Insome embodiments of the WLAN 10, the AP 14 and the SUT-capable stations25-1, 25-2, and 25-3 format PHY communication frames in the uplinkand/or downlink direction using the techniques described in the '603application.

FIG. 2A is a diagram of a prior art data unit 60 that the legacy clientstation 25-4 is configured to transmit to the AP 14 via OFDM modulation.The data unit 60 conforms to the IEEE 802.11a Standard and occupies a 20Megahertz (MHz) band. The data unit 60 includes a preamble having alegacy short training field (L-STF) 62, a legacy long training field(L-LTF) 64, and a legacy signal field (L-SIG) 66. The data unit 60 alsoincludes a data portion 68. The L-STF 62 generally includes informationthat is useful for packet detection and synchronization, whereas theL-LTF 64 generally includes information that is useful for channelestimation and fine synchronization. The L-SIG 66 generally signals PHYparameters to the receiving device.

FIG. 2B is a diagram of a prior art OFDM data unit 78 that the legacyclient station 25-4 is configured to transmit to the AP 14 using twospace-time streams for a data portion. The data unit 78 conforms to theIEEE 802.11n Standard, occupies a 20 MHz band, and is designed for mixedmode situations, i.e., when the WLAN includes one or more clientstations that conform to the IEEE 802.11a Standard but not the IEEE802.11n Standard. The data unit 78 includes a legacy preamble portionhaving an L-STF 80, an L-LTF 81, an L-SIG 82, and a high throughputsignal field (HT-SIG) 83. The data unit 78 also includes ahigh-throughput portion (shaded for clarity of illustration) having ahigh throughput short training field (HT-STF) 84, two data highthroughput long training fields (HT-LTFs) 85-1 and 85-2, and a dataportion 87.

FIG. 2C is a diagram of a prior art OFDM data unit 90 that the legacyclient station 25-4 is configured to transmit to the AP 14 using twospace-time streams for a data portion in one scenario in which theclient station 25-4 is capable of transmitting at least two space-timestreams. The data unit 90 conforms to the IEEE 802.11n Standard,occupies a 20 MHz band, and is designed for “Greenfield” situations,i.e., when the WLAN does not include any client stations that conform tothe IEEE 802.11a Standard but not the IEEE 802.11n Standard. The dataunit 90 includes a preamble having a high throughput Greenfield shorttraining field (HT-GF-STF) 91, a first high throughput long trainingfield (HT-LTF1) 92, a HT-SIG 93, a second high throughput long trainingfield (HT-LTF2) 94, and a data portion 95.

FIG. 2D is a diagram of a prior art OFDM data unit 100 that the legacyclient station 25-4 is configured to transmit to the AP 14 using fourspace-time streams for a data portion in one scenario in which theclient station 25-4 is capable of transmitting at least four space-timestreams. The data unit 100 conforms to the IEEE 802.11n Standard,occupies a 20 MHz band, and is designed for “Greenfield” situations,i.e., when the WLAN does not include any client stations that conform tothe IEEE 802.11a Standard but not the IEEE 802.11n Standard. The dataunit 100 includes a preamble having a high throughput Greenfield shorttraining field (HT-GF-STF) 101, a first high throughput long trainingfield (HT-LTF1) 102, a HT-SIG 103, and a block of three HT-LTF fields104-106, and a data portion 107.

FIG. 2E is a diagram of a prior art OFDM sounding data unit 110 that thelegacy client station 25-4 is configured to transmit to the AP 14 tosound two space-time streams in one scenario in which the client station25-4 is capable of transmitting at least two space-time streams. Thedata unit 110 conforms to the IEEE 802.11n Standard, occupies a 20 MHzband, and is designed for mixed mode situations, i.e., when the WLANincludes one or more client stations that conform to the IEEE 802.11aStandard but not the IEEE 802.11n Standard. The data unit 110 includes alegacy preamble portion having an L-STF 111, an L-LTF 112, an L-SIG 113,and a high throughput signal field (HT-SIG) 114. The data unit 110 alsoincludes a high-throughput portion having a high throughput shorttraining field (HT-STF) 115, two data high throughput long trainingfields (HT-LTFs) 116 and 117. The data unit 110 does not include a dataportion.

FIG. 2F is a diagram of a prior art OFDM data unit 120 that the legacyclient station 25-4 is configured to transmit to the AP 14 to sound twospace-time streams in one scenario in which the client station 25-4 iscapable of transmitting at least two space-time streams. The data unit120 conforms to the IEEE 802.11n Standard, occupies a 20 MHz band, andis designed for “Greenfield” situations, i.e., when the WLAN does notinclude any client stations that conform to the IEEE 802.11a Standardbut not the IEEE 802.11n Standard. The data unit 120 includes a preamblehaving a high throughput Greenfield short training field (HT-GF-STF)121, a first high throughput long training field (HT-LTF1) 122, a HT-SIG123, a second high throughput long training field (HT-LTF2) 124. Thedata unit 120 does not include a data portion.

It is noted that in some prior art networks, network devices use theformats discussed with reference to FIGS. 2A-F to transmit uplink datafrom a single transmitter to a receiver, and that space-time streams inthese devices are defined between Tx antennas and Rx antennas of a pairof devices. Further, these prior networks support at most fourspace-time streams to convey data in a data portion of the data unit, orto sound space-time streams for estimating the corresponding MIMOchannel. Receiving devices use at most one block of LTF fields includedin the data units 60, 89, 90, 100, 110, or 120 for data demodulation(e.g., the fields 104-106 of the data unit 100 illustrated in FIG. 2D).

Prior to describing an access protocol and PHY preamble formats thatSUT-capable stations use to simultaneously transmit uplink data, amathematical model that generally describes estimation of a MIMO channelusing a PHY preamble is briefly considered. A high throughput longtraining field (HT-LTF) is defined as a finite sequence of values (e.g.,“1, 1, −1, −1, 1, . . . 1, 1”). To train or estimate a MIMO channelhaving N_(STS) space-time streams, N_(STS) or more HT-LTFs aretransmitted to the receiver. For example, to enable data demodulationwhen a transmitter uses four space-streams to transmit a data payload(e.g., the data portion of a data unit) to a receiver, the transmitterin an embodiment generates four HT-LTFs and transmits the four HT-LTFsusing four different spatial mapping configurations. In someembodiments, a MIMO channel between a transmitter and a receiverincludes more than four space-time streams. In these cases, thetransmitter and the receiver can utilize the techniques and frameformats described in the in U.S. patent application Ser. No. 12/790,158,filed on May 28, 2010 and entitled “PHY Frame Formats in a System withmore than Four Space-Time Streams,” (“the '158 application”), the entiredisclosure of which is expressly incorporated by reference herein.

For each sub-carrier used in the OFDM mode, an instance of the HT-LTFsequence is mapped to a set of space-time streams using a matrixP_(HTLTF). In an embodiment, the matrix P_(HTLTF) is defined as thefollowing Hadamard matrix:

$P_{HTLTF} = \begin{bmatrix}1 & {- 1} & 1 & 1 \\1 & 1 & {- 1} & 1 \\1 & 1 & 1 & {- 1} \\{- 1} & 1 & 1 & 1\end{bmatrix}$

In this embodiment, each element of P_(HTLTF) is +1 or −1. In anotherembodiment, however, each element of P_(HTLTF) is a complex number(e.g., a Discrete Fourier Transform matrix is used as P_(HTLTF)). Inanother embodiment, some elements of P_(HTLTF) are integers other than+1 or −1.

As each HT-LTF is generated, a separate column of the matrix P_(HTLTF)is used to map the values to space-time streams. For example, the firstcolumn of the matrix P_(HTLTF) is applied to each value in the sequencedefining the first instance of HT-LTF in a certain PHY preamble, thesecond column of the matrix P_(HTLTF) is applied to each value in thesequence defining the second instance of HT-LTF of the PHY preamble,etc. In an embodiment, each instance of HT-LTF is an OFDM symbol.

In at least some of the embodiments, a frequency-domain Cyclic DelayDiversity (CDD) matrix D is applied to each of the resulting space-timestreams of the OFDM-MIMO channel to avoid undesirable beamformingeffects, for example. Application of the CDD matrix is equivalent tointroducing linear phase shifts over different sub-carriers of OFDM inat least some embodiments. Further, in accordance with an embodiment,the space-time streams are then mapped to transmit chains of thetransmitting device, each of which is associated with a correspondingtransmit antenna. In general, the number of transmit antennas N_(TX) isgreater than or equal to the number of space-time streams N_(STS). Tothis end, a spatial mapping matrix Q is defined at a transmitter.

In accordance with some protocols, the time-domain signal transmittedvia a transmit chain i_(TX) and received via N_(SR) sub-carriers overN_(STS) space-time streams is given by:

$\begin{matrix}{{r_{{HT} - {LTF}}^{n,i_{tx}}(t)} = {\frac{1}{\sqrt{N_{STS}N_{{HT} - {LTF}}^{Tone}}}{{w_{T_{{HT} - {LTF}_{n}}}(t)} \cdot {\sum\limits_{k = {- N_{SR}}}^{N_{SR}}{\sum\limits_{i_{STS} = 1}^{N_{STS}}{{\left\lbrack Q_{k} \right\rbrack_{i_{TX},i_{STS}}\left\lbrack P_{HTLTF} \right\rbrack}_{i_{STS},n}\gamma_{k}{{{HTLTF}(k)} \cdot {\exp\left( {{j2\pi}\; k\;{\Delta_{F}\left( {t - T_{GI} - T_{CS}^{i_{STS},n}} \right)}} \right)}}}}}}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

In Eq. 1, the exponential term corresponds to the phase shift using thefrequency CDD matrix D on the sub-carrier k for the space-time streami_(STS). The terms T_(GI) and T_(CS) ^(i) ^(STS) used in defining theexponential term of Eq. 1 are the same as or similar to the terms T_(GI)and T_(CS) defined by IEEE 802.11n Standard, the entire disclosure ofwhich is expressly incorporated by reference herein. Similarly, in anembodiment, the function γ_(k) is defined as in the IEEE 802.11nStandard or is another suitable function similar to the correspondingfunction defined in the IEEE 802.11n Standard.

Accordingly, for a sub-carrier k, the received signal x^(k)corresponding to all transmitted HT-LTFs is given by:x ^(k) =H ^(k) Q _(1:N) _(TX) _(,1:N) _(STS) ^(k) D _(N) _(STS) ^(k) P_(1:N) _(STS) _(,1:N) _(HTLTF) HTLTF(k)  (Eq. 2),

where H^(k) is a MIMO N_(RX)×N_(TX) channel matrix (with N_(RX)corresponding to the number of receive antennas and N_(TX) correspondingto the number of transmit antennas), Q_(1:N) _(TX) _(,1:N) _(STS) is thefirst N_(STS) columns of an antenna map or spatial mapping matrix thatmaps space-time streams to N_(TX) transmit antennas, D_(N) _(STS) ^(k)is a frequency-domain CDD matrix, P_(1:N) _(STS) _(,1:N) _(HTLTF) is thefirst N_(STS) rows of the matrix P, and HTLTF(k) is a pilot symbol usedin the k-th sub-carrier.

In accordance with an embodiment of the present disclosure, two or moreSUT-capable stations jointly transmit several training fields, referredto hereinafter as very high throughput short training fields (VHT-STFs)and very high throughput long training fields (VHT-LTFs), to train aMIMO channel defined by the Rx antenna array of an AP and a virtual Txantenna array of the two or more SUT-capable stations. Depending on theembodiments, the total number of VHT-(S)LTFs is two, four, five, six,etc. In a system with U STA-capable stations,

P_(i_(STS_(u)), n)^((k))specifies a training (or “pilot”) sequence transmitted on thesub-carrier k for the n-th training symbol at the i_(STS)-th space-timestream of station u. Each of the U STA-capable stations transmits thecorresponding training sequence over N_(STS) _(—) _(u) space-timestreams. For example, station 1 transmits over two space-time streams,station 2 transmits over four space-time streams, etc. In theseembodiments, a channel training matrix P^((k)) how pilot sequences aredistributed during MIMO OFDM training over U stations. In particular,according to this embodiment, each row of the channel training matrixP^((k)) is transmitted over a single space-time stream, and each columnof the matrix P^((k)) to one VHT-(S)LTF symbol (i.e., one OFDM symbol).Thus, the overall set of VHT-(S)LTFs transmitted by U SUT-capablestations, prior to spatial mapping, is given by P^((k))s(k), where s(k)is the training symbol at the k-th sub-carrier.

In an embodiment, N_(STS) _(—) _(u) rows of the matrix P^((k)) areallocated to each station u in the set of U SUT-capable stations.Accordingly, VHT-(S)LTFs transmitted from the station u is P_((u))^((k))s(k). Thus,

$\begin{matrix}{P^{(k)} = \begin{bmatrix}P_{(1)}^{(k)} \\P_{(2)}^{(k)} \\\vdots \\P_{(U)}^{(k)}\end{bmatrix}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

In general, the matrix P^((k)) is any matrix known at the AP and theSUT-capable stations. In some embodiments, the matrix is P^((k))orthogonal. In an embodiment, the matrix is P^((k)) unitary. In anembodiment, is P^((k)) given by

$\begin{matrix}{{P^{(k)} = {{diag}{\left\{ {a_{1}^{(k)}\mspace{14mu}\ldots\mspace{14mu} a_{STS\_ SUT}^{(k)}} \right\}\left\lbrack P_{VHTLTF} \right\rbrack}}},} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

where P_(VHTLTF) is a row-orthogonal matrix and a, is any complexnumber. In an embodiment, P_(VHTLTF) is similar or identical to thematrix P_(HTLTF) discussed above. The techniques for assigning rows ofthe matrix P^((k)) to SUT-capable stations from several stations arefurther considered below, and several examples of using a channeltraining matrix when transmitting VHT-LTFs in parallel are discussedwith reference to FIGS. 6A-C.

FIG. 3 is a diagram illustrating communications in a WLAN 150, accordingto an embodiment, during three time periods: a first carrier sense,multiple access (CSMA) period 154, an SUT time period 156, and a secondCSMA period 158. In FIG. 3, time progresses from left to right so thatthe first CSMA period 154 occurs first, the SUT time period 156 occurssecond, and the second CSMA period 158 occurs third. In this embodiment,the WLAN includes an AP, a plurality of legacy client stations (LCs),and a plurality of SUT-capable client stations (SC).

According to the IEEE 802.11a and the IEEE 802.11n Standards, differentdevices share the communication channel by utilizing a CSMA protocol.Generally speaking, CSMA, according to the IEEE 802.11a and the IEEE802.11n Standards, specifies that a device that wishes to transmitshould first check whether another device in the WLAN is alreadytransmitting. If another device is transmitting, the device should waitfor a backoff time period and then again check again to see whether thecommunication channel is being used. If a device detects that thecommunication channel is not being used, the device then transmits usingthe channel. With CSMA, in other words, data that is for a particulardevice (i.e., not multicast data) can only be transmitted on the channelwhen no other data is being transmitted on the channel.

In the first CSMA period 154, the AP transmits a legacy downlink signal(e.g., a communication frame, a data packet, another type of a dataunit) to one of the LCs. In general, the AP and the LC can negotiate theCSMA period 154 using any suitable handshaking protocol such asrequest-to-send/clear-to-send (RTS/CTS). The SUT period 156 is reservedfor simultaneous uplink transmissions from two or more SUT-capablestations to the AP. Thus, in the SUT period 156, several SCs transmituplink signals with independent data streams to the AP at the same time.In the second CSMA period 158, an LC transmits a legacy uplink signal tothe AP.

Embodiments of an SUT access protocol for SUT signals will now bedescribed. In the following embodiments, the AP is equipped with anantenna array (i.e., a plurality of antennas) and a MIMO receiver. TheAP and/or client stations reserve a protected SUT time period forsimultaneous uplink transmissions for use by SUT-capable stations only,and accordingly exclude legacy stations from using the SUT period foruplink transmissions. To this end, an AP and/or stations can use anysuitable MAC-layer scheduling/messaging mechanisms, includingscheduling/messaging mechanisms similar to those defined in the existingIEEE 802.11a/n Standards. During an SUT period, signals from several SCsare synchronized using explicit or implicit scheduling. In someembodiments, an AP generates synchronization information and transmitsthe synchronization information to the SCs prior to receiving SUTsignals.

FIG. 4A is a timing diagram of an SUT period, according to anembodiment. An AP allocates a protected SUT period 200 of duration Tu.In the illustrated embodiment, the AP controls synchronization andtiming of the SUT period 200 at least partly by generating asynchronization frame 202 and broadcasting (or multicasting) thesynchronization frame 202 to SUT-capable stations. In anotherembodiment, the AP transmits a synchronization frame to each SUT-capablestation individually. The SUT period 200 begins following thetransmission of the synchronization frame 202 after a synchronizationperiod of duration Td. The synchronization frame 202 specifies timing ofthe SUT period 200 (e.g., the start time, the duration Tu), according toan embodiment. Further, depending on the particular embodiment, thesynchronization frame 202 includes one or more of the following: anindication of which of the SUT-capable stations are expected to transmituplink data during the SUT period 200, a respective index assigned toeach SUT-capable station expected to communicate during the SUT period200, the total number of space-time streams available for uplinktransmission, a listing of space-time streams allocated to eachindividual SUT-capable station for the SUT period 200, bandwidth of SUTframes (e.g., 20 MHz, 40 MHz, 80 MHz), a respective modulation andcoding scheme for each SUT-capable station expected to transmit duringthe SUT period 200, a respective power control parameter for eachSUT-capable station expected to transmit during the SUT period 200, themaximum duration of an SUT frame that can be transmitted during the SUTperiod 200, etc.

In some embodiments, the AP controls the selection of SUT-capablestations that transmit uplink data during the SUT period 200. As oneexample, the AP determines, based on previous communications, that fourSUT-capable stations wish to transmit communication frames to the AP inthe next available SUT period, but that only three of the fourSUT-capable stations can be accommodated during the SUT period 200. Inan embodiment, the AP selects three SUT-capable stations based on thequality of service or another suitable criterion (or several criteria).The synchronization frame 202 subsequently includes a listing ofSUT-capable stations selected to communicate during the SUT period 200,according to an embodiment.

In the embodiment of FIG. 4A, N SUT-capable stations transmitcommunication frames or packets 210-1, 210-2, . . . 210-N, respectively.The frames 210-1, 210-2, . . . 210-N need not be of the same duration.However, as discussed in more detail below, the communication frames210-1, 210-2, . . . 210-N, in an embodiment, transmit identicalsequences in a synchronized manner during at least a portion of the PHYpreamble. Further, the frames 210-1, 210-2, . . . 210-N need not arriveat the AP at precisely the same time. However, the start of each of theframes 210-1, 210-2, . . . 210-N must be received within the durationTcp of a cyclic prefix (CP) of an OFDM symbol to ensure properprocessing, according to an embodiment. Thus, even though the SUT frame210-2 in FIG. 4 arrives later than the frames 210-1 and 210-N, the APcan properly process the SUT frame 210-2 because the start of the SUTframe 210-2 is received within the time interval of duration Tcp.

In an embodiment, each SUT-capable station includes a corresponding SUTindex (SUT_IDX) assigned to the station in the synchronization frame202. In another embodiment, the AP relies on the MAC address or anotherparameter to differentiate between the frames 210-1, 210-2, . . . 210-N.To acknowledge a successful receipt of N SUT frames, the AP transmits Nacknowledgement frames 210-1, 210-2, . . . 210-N separated by apredefined Short Inter Frame Space (SIFS), according to an embodiment.The technique for transmitting acknowledgements illustrated in FIG. 4may be referred to as “staggered acknowledgement.” Each acknowledgmentis a positive acknowledgement (ACK) or a negative acknowledgement(NACK), depending on whether the SUT frame has been received properlyand/or timely.

In an embodiment, the AP schedules Tu so as to allocate both SUT packetsand staggered acknowledgements. Further, according to an embodiment, thetransmissions of acknowledgements are pre-scheduled. For example, the APmay schedule to transmit an ACK or a NACK for an SUT frame to bereceived from a station with SUT_IDX=1 at a time Ta irrespective of theactual duration of the SUT frame. The acknowledgements are transmittedin the ascending order of the SUT_IDX, in an embodiment.

Referring to FIG. 4B, an SUT period 230 is generally similar to the SUTperiod 200 discussed with reference to FIG. 4A, except that uponreceiving SUT frames 232-1, 232-2, . . . 232-N, an AP transmits a blockacknowledgement frame 234 to SUT-capable stations that communicateduring the SUT period 230. In an embodiment, the block acknowledgementframe 234 specifies the identity (e.g., SUT_IDX or the MAC address) ofeach SUT-capable station from which a SUT frame has been successfullyreceived. In one such embodiment, the block acknowledgement frame 234 istransmitted after a predefined interval (e.g., SIFS) following thereceipt of the longest SUT frame.

In some embodiments, an AP specifies a fixed duration of SUT frames in asynchronization frame, for example. In one such embodiment, SUT-capablestations pad the corresponding SUT frames at the PHY layer or the MAClayer. FIG. 4C illustrates one such padding technique. During an SUTperiod 240, SUT frames 242-1, 242-2, and 242-N are padded to conform tothe same length requirement.

However, in accordance with another embodiment, the AP checks theduration of each SUT frame using a station-specific field such as HT-SIGor VHT-SIG in the PHY preamble, for example, and delays clear channelassessment (CCA) until the longest SUT frame is fully received. Sharedas well as station-specific fields of a PHY preamble are discussed morefully below. In another embodiment, an AP determines certain PHYparameters, such as MCS and the length of an SUT packet, for eachSUT-capable station and provides PHY parameters to the stations in asynchronization frame. In this case, stations need not specify thealready-determined PHY parameters in a HT-SIG or VHT-SIG field of thepreamble. In an embodiment, stations omit a VHT-SIG from the PHYpreamble when PHY parameters are determined by the AP prior to stationstransmitting SUT frames.

As indicated above, an AP in at least some of the embodiments controlsthe number of available space-time streams and allocates space-timestreams to individual SUT-capable stations. In an example scenario, anAP is capable of receiving data over four concurrent space-time streamsvia four receive antennas (i.e., Nrx=Nss 4). The AP excepts to receiveSUT frames from two SUT-capable stations A and B during an SUT period(e.g., 200, 230, 240). In an embodiment, the AP assigns two space-timestreams to station A and two space-time streams to station B. For thispurpose, the AP assigns the SUT_IDX=1 to station A and SUT_IDX=2 tostation B, and includes an indication in the synchronization frame thatstation A is being assigned two space-time streams, and station B isbeing assigned two space-time streams, according to an embodiment. Eachof the AP and stations A and B stores the same four-by-four channelestimation matrix P^((k)) each sub-carrier k, and station A accordinglyutilizes a first set of two rows of the matrix P^((k)) (e.g., the firsttwo rows) to map data to space-time streams, while station B utilizes asecond set of two rows of the matrix P^((k)) (e.g., the last two rows).Because the AP stores the assignment of space-time streams and SUTindexes to stations, the AP can properly separate signals received fromstations A and station B, respectively. In an embodiment, the APtransmits to Stations A and B indications of the set of rows of thematrix P^((k)) that are to be utilized by the Stations A and B.

In an embodiment, an AP monitors signal strength, interference levels,and other physical parameters of SUT-capable stations to controltransmission power at each station. In other words, because the abilityof a client station to transmit SUT data to an AP is dependent both onthe properties of the channel and the power at which other stationstransmit SUT data within the same time period, an AP in some embodimentsdetermines proper MCSs and power parameters for the participatingSUT-capable stations upon conducting interference cancellation. In anembodiment, the AP suggests, rather than mandates, station-specific MCSvalues in the synchronization frame.

In another embodiment, each SUT-capable station independently determinesthe proper MCS using slow adaptation, for example. In particular, astation in some embodiments checks whether the rate used in a certainSUT period was too high or too low (based on the packer error rate, forexample), and adjusts the MCS in the next SUT period.

Several example formats of a PHY preamble of an SUT frame are discussednext. In some embodiments, SUT-capable stations format SUT framesaccording to these formats to enable the AP to accurately estimate andtrain the MIMO channel as well as multiplex (i.e., separate)station-specific information received at the same time via thecorresponding antenna array. For clarity, overall formats are discussedfirst with reference to FIGS. 5A-C, and several embodiments of specificportions of a PHY preamble are then discussed with reference to FIGS.6A-C. SUT-capable stations can utilize these formats during SUT periods(e.g., the periods 200, 230, or 240).

FIG. 5A is a diagram of communication frames 302-1, 302-2, . . . 302-Nthat SUT-capable stations are configured to transmit to an AP at thesame time, in accordance with an embodiment. The frames 302-1, 302-2, .. . 302-N are consistent with a format 300 and are designed for mixedmode situations, i.e., when the WLAN includes a legacy station thatconforms to the IEEE 802.11a Standard and/or the IEEE 802.11n Standardbut not to a protocol that specifies SUT transmissions. In accordancewith the format 300, a communication frame includes a legacy portion 310and a VHT portion 312. The PHY preamble of the communication frameincludes the legacy portion 310 and one or several fields of the VHTportion 312. The legacy portion 310 is the same in each of the PHYpreambles 302-1, 302-2, . . . 302-N. By contrast, the VHT portion 312 ofthe PHY preambles 302-1, 302-2, . . . 302-N includes station-specificinformation, and thus need not be the same. The legacy portion 310 istransmitted in accordance with such data rate and MCS that all receiversoperating in a WLAN, i.e., legacy receivers and non-legacy receivers,are capable of properly demodulating the information included in thisportion. However, only some receivers (e.g., SUT-capable APs andstations) are capable of properly receiving information included in theVHT portion 312.

In an embodiment, the L-SIG field and/or the HT-SIG field in the legacyportion 310 specify the duration of the communication frame so that alegacy station can properly determine the duration of the correspondingcommunication frame. In this manner, the legacy station does not attemptto decode SUT-DATA or transmit during the VHT portion 312. In someembodiments, the L-SIG field or the HT-SIG field indicates the durationTu corresponding to the length of the SUT period. In one suchembodiment, the duration Tu is previously specified in thesynchronization frame transmitted by the AP, as discussed above. Inanother embodiment, the L-SIG field or the HT-SIG field specifies themaximum duration of a communication frame consistent with the format300.

With continued reference to FIG. 5A, depending on the embodiment, thelegacy portion 310 includes zero, one, or several HT-SIG fields. Inanother embodiment, the information associated with HT-SIG fields isinstead included as the first block of the VHT-SIG field. If an HT-SIGis included, according to some embodiments, a station uses thetechniques for “spoofing” an IEEE 802.11n receiver or IEEE 802.11a/creceiver, as described in the '603 application incorporated by referenceabove.

FIG. 5B illustrates communication frames 332-1, 332-2, . . . 332-N thatSUT-capable stations are configured to transmit to an AP at the sametime, in accordance with another embodiment. The frames 332-1, 332-2, .. . 332-N are consistent with a format 330 and are designed for“Greenfield” situations, i.e., when the WLAN does not include any clientstations that do not support SUT transmissions. In another scenario, anAP explicitly reserves an SUT time period for use by SUT-capablestations only using a synchronization frame or a suitable MAC-layerscheduling mechanism, for example, and thus does not require anyinformation to be transmitted according to a legacy protocol during theSUT period. Accordingly, unlike the format 300, the format 330 specifiesonly a VHT portion.

Referring to FIG. 5C, a frame format 350 is generally similar to theformat 300, except that a VHT-SIG field in the last portion of the PHYpreamble is omitted, according to another embodiment. In an embodiment,stations utilize the frame format 350 during an SUT period when an APspecifies the relevant PHY parameters in a synchronization frame, andthe stations need not “echo” the same parameters back to the AP.

As indicated above, the transmit antennas of a group of SUT-capablestations can be regarded as a large virtual Tx antenna array.Accordingly, a larger number of training fields (LTFs) is needed totrain a MIMO channel defined by the Rx antenna array of an AP and thevirtual Tx antenna array. For example, four LTFs are needed to train avirtual Tx antenna array defined by two pairs of antennas of stations Aand B, respectively.

Referring to FIG. 6A, several SUT-capable stations transmit a sequenceof VHT-LTFs conforming to a format 400 as a part of the VHT portion of aframe (e.g., the VHT portion 312 or 330), according to an embodiment.Each of the stations 1, 2, . . . U transmits the same number (N_(STS)_(—) _(SUT)) of VHT-LTFs to jointly train a large MIMO channel. As oneexample, N_(STS) _(—) _(SUT) can be eight. In general, N_(STS) _(—)_(SUT) is equal to the total number of space-time streams of the MIMOchannel between the U SUT-capable stations and an AP. Thus,

$\begin{matrix}{{N_{STS\_ SUT} = {\sum\limits_{k}N_{STS\_ k}}},} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

where N_(STS) _(—) _(k) corresponds to the number of space-time streamsused by station k.

As indicated above, an AP specifies the total number of space-timestreams as well as assignments of rows of the channel estimation matrixP^((k)) to SUT-capable stations, according to an embodiment. During anSUT period, a station applies the one or more rows of a shared P^((k))matrix that have been assigned to the station in a synchronization frame(or using another assignment mechanism). For example, station 1transmits the sequence 402-1 using rows 1 and 2 of the shared P^((k))matrix, station 2 transmits the sequence 402-2 using rows 3 and 4 of theshared P^((k)) matrix, etc. Upon receiving, at substantially the sametime, the sequences 402-1, 402-2, . . . 402-U, the AP estimates the MIMOchannel using the matrix P^((k)) in view of the assignment of space-timestreams and indexes (STS_IDX) to individual stations. In an embodiment,the rows of the P^((k)) matrix are allocated to each station accordingto the SUT_IDX, e.g., N_(STS) ⁽¹⁾ rows are allocated to a station withthe index SUT_IDX=1, N_(STS) ⁽²⁾ rows are allocated to a station withthe index SUT_IDX=2, etc. In this manner, the AP can receive a combinedSUT signal from several stations, “undo” the P^((k)) matrix (which isknown to both the AP and all stations participating in the SUT period),and unambiguously determine which space time streams correspond to whichstations. Subsequently, when processing the VHT-SIG and/or data portionsof the communication frames, the AP can separate (“demultiplex”)station-specific information (e.g., data received on space-time streams1 and 2 is from station 1, data received on space-time streams 3-6 isfrom station 2, etc.).

As will be understood, for the n-th VHT-LTF received from the station u,the signal transmitted from the i_(TX) ^((u))-th antenna is given by

$\begin{matrix}{{r_{{VHT} - {LTF}}^{n,i_{TX}^{(u)}}(t)} = {\frac{1}{\sqrt{N_{STS}N_{{VHT} - {LTF}}^{Tone}}}{{w_{T_{{VHT} - {LTF}_{n}}}(t)} \cdot {\sum\limits_{k = {- N_{SR}}}^{N_{SR}}{\sum\limits_{i_{STS} = 1}^{N_{STS}}{{\left\lbrack Q_{k}^{(u)} \right\rbrack_{i_{TX}^{(u)},i_{STS}}\left\lbrack P_{VHTLTF}^{(u)} \right\rbrack}_{i_{STS},n}\gamma_{k}{{{VHTLTF}(k)} \cdot {\exp\left( {{j2\pi}\; k\;{\Delta_{F}\left( {t - T_{GI} - T_{CS}^{i_{STS},{(u)}}} \right)}} \right)}}}}}}}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

in accordance with an embodiment. The matrix P_(VHTLTF) in someembodiments is defined similar to the four-by-four matrix P_(HTLTF)discussed above. In general, the number of rows of the P_(VHTLTF) matrixis equal to N_(STS) _(—SUT) , and the number of columns of theP_(VHTLTF) matrix is equal to the number of VHT-LTFs. Further, accordingto some embodiments, the matrix P_(VHTLTF) is extended to account formore space-time streams or, conversely, is reduced to account for lessthan four space-time streams. For example, a larger P_(VHTLTF) matrixcan be defined according to the '158 reference incorporated above. Asmaller P_(VHTLTF) matrix can be defined as a sub-matrix of afour-by-four P_(VHTLTF) matrix.

In an embodiment, upon simultaneously receiving N VHT-LTFs signals fromU stations, the AP applies the following model for each sub-carrier k toestimate the channel:

$\begin{matrix}\left\lbrack {{\begin{matrix}{H^{k,{(1)}}Q_{k}^{(1)}D_{k}^{(1)}} & {H^{k,{(2)}}Q_{k}^{(2)}D_{k}^{(2)}} & \ldots & {\left. {H^{k,{(U)}}Q_{k}^{(U)}D_{k}^{(U)}} \right\rbrack{{\quad\quad}\begin{bmatrix}P_{VHTLTF}^{(1)} \\P_{VHTLTF}^{(2)} \\\vdots \\P_{VHTLTF}^{(U)}\end{bmatrix}}}\end{matrix} \cdot {\quad\quad}}{\quad\quad}{{VHTLTF}(k)}} \right. & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

Accordingly, the AP “undoes” (i.e., cancels out) the pre-definedsequence VHTLTF(k), inverts the matrix P_(VHTLTF), and arrives at anestimate of the MIMO channel according to Eq. 8 provided below:H _(EST) ^(k) [H ^(k,(1)) Q _(k) ⁽¹⁾ D _(k) ⁽¹⁾ H ^(k,(2)) Q _(k) ⁽²⁾ D_(k) ⁽²⁾ . . . H ^(k,(U)) Q _(k) ^((U)) D _(k) ^((U))]  (Eq. 8)

In another embodiment, stations use a staggered approach to estimate theMIMO channel for SUT communications. In other words, rather thantraining the MIMO channel concurrently, two or more stationssequentially transmit training data so that the AP can estimaterespective space-time streams. Of course, once the MIMO channel isproperly estimated, an AP can simultaneously receive station-specificdata during an SUT period.

FIG. 6B is a timing diagram that illustrates an example staggeredapproach to estimate a MIMO channel for SUT communications. Stations 1,2, . . . U transmit PHY preambles having portions 402-1, 402-2, . . .402-U. In an embodiment, station 1 transmits a short training fieldVHT-STF followed by two long training fields, while stations 2-U remainidle, i.e., do not transmit data to the AP. Next, station 2 transmit aVHT-STF followed by a VHT-LTF, while all other stations are idle. Acycle of staggered MIMO channel estimation for SUT communicationscompletes when station U transmits a VHT-STF followed by four VHT-LTFswhile all other stations are idle, according to the example scenario ofFIG. 6B.

In an embodiment, each station transmits at least as many VHT-LTFs asspace-time streams assigned to the station by the AP. In someembodiments, the station use the assigned rows of a P_(VHTLTF) (orP^((k))) matrix when transmitting VHT-LTFs. Similar to the examplediscussed above, in an embodiment, stations transmit training fields inthe order of SUT indexes assigned in the synchronization frame, so thatthe AP can unambiguously match the received VHT-LTFs to particularstations based on the order or timing of the received VHT-LTFs. Further,a station calculates the duration of the idle period(s) based on thetotal number of space-time streams N_(STS) _(—) _(SUT) announced in thesynchronization frame, the SUT index and, in some cases, the assignmentto space-time streams to other stations, according to an embodiment. Forexample, station 2 determines the time when station should transmitVHT-STF based on the number of space-time streams assigned to station 1.

According to an embodiment, each station separately transmits theVHT-STF field for automatic gain control (AGC) adjustment. Further, inan embodiment, the stations subsequently transmit another VHT-STF at thesame time. In an embodiment, the stations use the first column of theP^((k)) matrix to jointly transmit a VHT-STF.

If a staggered training technique such as the one illustrated in FIG. 6Bis used, an AP applies, according to an embodiment, the following modelthat describes the received VHT-LTFs at a sub-carrier k:H ^(k,(1)) Q _(k) ⁽¹⁾ D _(k) ⁽¹⁾ VHTLTF(k), H ^(k,(2)) Q _(k) ⁽²⁾ D _(k)⁽²⁾ VHTLTF(k), . . . , H ^(k,(U)) Q _(k) ^((U)) D _(k) ^((U))VHTLTF(k)  (Eq. 9)

Upon canceling out the pre-defined sequence VHTLTF(k), P_(VHTLTF)inversion, etc., the AP arrives at an estimate of the MIMO channelaccording to Eq. 10 provided below:H _(EST) ^(k)=[β₁ H ^(k,(1)) Q _(k) ⁽¹⁾ D _(k) ⁽¹⁾ β₂ H ^(k,(2)) Q _(k)⁽²⁾ D _(k) ⁽²⁾ . . . β_(U) H ^(k,(U)) Q _(k) ^((U)) D _(k) ^((U))]  (Eq.10)

In some embodiments, an AP compensates for the differences in transmitpower and AGC settings that may result in column-wise distortion byscaling some of the sub-matrices (e.g., H^(k,(1))Q_(k) ⁽¹⁾D_(k) ⁽¹⁾). Inan embodiment, the AP considers this distortion factor when implementinginterference cancellation and/or data demodulation algorithms.

Referring to FIG. 6C, stations 1, 2, and 3 in another embodiment utilizea mixed technique that includes both simultaneous VHT-LTF transmissionsand staggered VHT-LTF transmissions. Stations 1 and 2 simultaneouslytransmit a group of four VHT-LTFs in frames 410-1 and 410-2,respectively, while station 3 is idle. Together, stations 1 and 2 form afirst group of stations. Next, station 3 transmits four VHT-LTFs in aframe 410-3 while stations 1 and 2 are idle. In this example, station 3forms a second group of stations. In an embodiment, an AP announces thegrouping of stations in a synchronization frame. Using the groupinginformation, stations can calculate the corresponding idle periods. Inan embodiment, each group of stations sends a group-specific VHT-STF forAGC adjustment.

In the example FIG. 6C, an AP applies the following model that describesthe received VHT-LTFs at a sub-carrier k:

$\begin{matrix}\left\lbrack {{{\begin{matrix}{H^{k,{(1)}}Q_{k}^{(1)}D_{k}^{(2)}} & \left. {H^{k,{(2)}}Q_{k}^{(2)}D_{k}^{(2)}} \right\rbrack & {{\quad\quad}\begin{bmatrix}P_{VHTLTF}^{(1)} \\P_{VHTLTF}^{(2)}\end{bmatrix}}\end{matrix} \cdot {\quad\quad}}{\quad\quad}{{VHTLTF}(k)}},\mspace{20mu}{and}} \right. & \left( {{{Eq}.\mspace{14mu} 11}A} \right) \\{H^{k,{(3)}}Q_{k}^{(3)}D_{k}^{(3)}{{VHTLTF}(k)}} & \left( {{{Eq}.\mspace{14mu} 11}B} \right)\end{matrix}$

According, the AP estimates the channel according to Eq. 12:H _(EST) ^(k)=[β₁ H ^(k,(1)) Q _(k) ⁽¹⁾ D _(k) ⁽¹⁾ β₁ H ^(k,(2)) Q _(k)⁽²⁾ D _(k) ⁽²⁾ . . . β₃ H ^(k,(3)) Q _(k) ⁽³⁾ D _(k) ⁽³⁾]  (Eq. 12)

As discussed above, in some cases stations provide station-specific PHYparameters to the AP in a VHT-SIG field. Several techniques forgenerating a VHT-SIG field are discussed next with reference to FIGS. 7and 8.

Referring first to FIG. 7, in an embodiment, a station modulates aVHT-SIG field 420 of a communication frame 422 with binary phase-shiftkeying (BPSK), binary convolution codes (BCC), and one space-time stream(i.e., N_(STS)=1). When an AP simultaneously receives several instancesof the VHT-SIG field 420 from several stations, the AP multiplexes thereceived signal to interpret and process station-specific PHYparameters. In an embodiment, each station uses the first column of thecorresponding spatial mapping matrix Q to spatially map the VHT-SIGfield 420. Referring back to FIGS. 5A-C, stations can similarly applythe first column of the spatial mapping matrix to the field VHT-SIG2 inthe corresponding VHT portion 312 or 330. In the embodiments, thereceived VHT-SIG field is given by:

$\begin{matrix}{{r_{{VHT} - {SIG}}^{i_{TX}^{(u)}}(t)} = {\frac{1}{\sqrt{N_{{VHT} - {SIG}}^{Tone}}}{{w_{T_{{VHT} - {SIG}}}(t)} \cdot {\sum\limits_{k = {- N_{SR}}}^{N_{SR}}{\left\lbrack Q_{k}^{(u)} \right\rbrack_{i_{TX}^{(u)},1}\gamma_{k}{{{VHTLTF}(k)} \cdot {\exp\left( {{j2\pi}\; k\;{\Delta_{F}\left( {t - T_{GI} - T_{CS}^{i_{STS},{(u)}}} \right)}} \right)}}}}}}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

With continued reference to FIG. 7, a data portion 424 of thecommunication frame 422 in some cases is transmitted over more than onespace-time stream. For example, in an embodiment, the number ofspace-time streams N_(STS) is specified in the synchronization framefrom the AP.

In an embodiment, to properly adjust AGC, the short training fieldVHT-STF 426 is transmitted with N_(STS)=1 using the element in the firstrow, first column of the P^((k)) and VHT-STF 428 is transmitted withN_(STS) N_(STS) _(—) _(DATA) using the first column of the P^((k))matrix. In some embodiments, the VHT-STF 426 (i.e., a short trainingfield that precedes the VHT-SIG) is omitted when a sequence of VHT-LTFsis followed by a VHT-STF field (see FIGS. 6A-C, for example).

In an embodiment, a VHT-STF transmitted with N_(STS)=N_(STS) _(—)_(DATA) is mapped to N_(STS) space-time streams using the first or thelast column of the P^((k)) matrix, and the resulting space-time streamsare mapped to N_(TX) transmit antennas using the Q matrix. The receivedVHT-SIG accordingly is given by:

$\begin{matrix}{{r_{{VHT} - {SIG}}^{i_{TX}^{(u)}}(t)} = {\frac{1}{\sqrt{N_{STS}^{(u)}N_{{VHT} - {SIG}}^{Tone}}}{{w_{T_{{VHT} - {SIG}}}(t)} \cdot {\sum\limits_{k = {- N_{SR}}}^{N_{SR}}{\sum\limits_{i_{STS} = 1}^{N_{STS}^{(u)}}{{\left\lbrack Q_{k}^{(u)} \right\rbrack_{i_{TX}^{(u)},i_{STS}}\left\lbrack P_{VHTLTF}^{(u)} \right\rbrack}_{i_{STS},n}\gamma_{k}{{{VHTSIG}( k)} \cdot {\exp\left( {{j2\pi}\; k\;{\Delta_{F}\left( {t - T_{GI} - T_{CS}^{i_{STS},{(u)}}} \right)}} \right)}}}}}}}} & \left( {{Eq}.\mspace{14mu} 14} \right)\end{matrix}$

Further, it is noted that an AP in this case can expect little powerfluctuation between a VHT-SIG field and a data field (e.g., the fields428 and 424, respectively), and thus no additional VHT-SIG field isneeded at least in some implementations and/or scenarios.

Referring to FIG. 8, a communication frame includes a portion 440 inwhich a VHT-SIG field 442 is mapped to N_(STS) streams using the first(P₁) column of the matrix P^((k)). Accordingly, the AP (or anotherreceiver) applies the vector P₁ when processing VHT-SIGs received fromdifferent stations.

In another embodiment, a VHT-SIG field is modulated using BPSK using thesame number of space-time streams as applied to the data portion of theframe. In this case, stations need not transmit another VHT-STF field.In an embodiment, if two OFDM symbols are required to modulate theVHT-SIG field over one space-time stream, a single OFDM symbol is usedto modulate the VHT-SIG field over two or more space-time streams.Further, because the AP knows the number of space-time streams allocatedto a particular station, the AP can automatically detect the modulationscheme.

Now referring to FIG. 9, an example communication frame generator 450operates in the network interface 27 of the client 25-1, for example, orin another SUT-capable client station. In an embodiment, some componentsof the frame generator 450 can be included in the MAC unit 28, whileother components of the frame generator 450 can be included in the PHYunit 29. The communication frame generator 450 generates communicationframes so as to communicate within an SUT time period according to aprotocol discussed with reference to FIGS. 4A-C, for example. In anembodiment, these frames conform to one of the overall formatsillustrated in FIGS. 5A-C. Further, in an embodiment, the specificfields or groups of fields (e.g., VHT-LTF sequences, VHT-SIG, etc.) inthese frames are consistent with the examples of FIG. 6A-C, 7, or 8.

In an embodiment, the frame generator 450 includes a PHY preamblegenerator 452 having a VHT-LTF controller 452-1 to produce symbols of along training field, a VHT-STF controller 452-2 to produce sequences ofa short training field, and a VHT-SIG controller 452-2 to produce afield that signals PHY parameters to another device (e.g., an AP). ThePHY preamble generator 452 supplies VHT-STF, VHT-LTF, and VHT-SIG fieldsto a P matrix mapper 454 that maps symbols to space-time streams. The Pmatrix mapper 454 applies a mapping scheme in accordance with aselection signal generated by a space-time stream selector 456. In thismanner, the space-time stream selector enables the station to operate asan SUT-capable station and transmit uplink data to an AP simultaneouslywith other stations. In an embodiment, the space-time stream 456 selectsparticular rows of a matrix P^((k)) to be applied to a field of a PHYpreamble. In an embodiment, the space-time stream selector 456 selectsthe rows of the matrix P^((k)) in view of the information supplied in asynchronization and processed by a synchronization frame processor 455.

The outputs of the P matrix mapper 454 are coupled to respective inputsof a CDD generator 458 that applies frequency cyclic delay diversityvalues to the corresponding space-time streams. In an embodiment, theblock controller 456 additionally controls the selection of values bythe CDD generator 458. In an embodiment, the CDD generator 458additionally receives a data payload via a line 462 from a data portiongenerator 463.

The outputs of the CDD generator 458 are coupled to a Q matrix mapper460 that performs spatial mapping of space-time streams to transmitchains 466, each including at least a respective transmit antenna.Similar to the P matrix mapper 454, the Q matrix mapper 460 iscommunicatively coupled to the space-time stream selector 456 thatselects an appropriate matrix Q for various fields of the communicationframes.

FIG. 10 illustrates an example SUT communication frame processor 500that operates in the AP 14 or a similar device configured to receivecommunication frames. The SUT communication frame processor 500 includesa legacy portion processor 502 and a VHT portion processor 504. Todemultiplex (separate) station-specific data transmitted in severalconcurrent communication frames, a data demultiplexer 506 appliesappropriate vectors to the received signal. In an embodiment, the datademultiplexer 506 cooperates with a P matrix controller 510 and/or asynchronization controller 512. In particular, the P matrix controller510 and the synchronization controller 512 in some embodiments allocatespace-time streams to individual stations, assign SUT indexes tostations, control the timing of an SUT time period, etc. Thus, forexample, the data demultiplexer 506 retrieves the mapping of stations toindividual space-time streams or groups of space-time streams of a MIMOchannel used in SUT communications from the synchronization controller512, retrieves the appropriate shared P^((k)) matrix from the P matrixcontroller 510, and uses this information to demultiplex the receiveddata.

In an embodiment, the communication frame processor 500 iscommunicatively coupled to an acknowledgment generator 520 thatgenerates ACKs or NACKs individually or according to a block format (seeFIGS. 4A-C and the corresponding discussion).

Referring to FIG. 11, a method 600 for scheduling a SUT time period andcommunicating with several SUT-capable stations within the SUT period isimplemented by the synchronization frame controller 512 or by a similarcomponent of an AP, according to an embodiment. At block 602, a timeperiod for SUT communications is scheduled in view of such factors asclient station compatibility, client station capabilities, bandwidthrequirements, etc. The AP excludes legacy stations from the SUT timeperiod and allows only SUT-capable stations during this period.Depending on the embodiment, the AP sets the duration of intervals Tu,Td, etc. In an embodiment, the AP determines the maximum length of a SUTcommunication frame. Upon scheduling an SUT time period, the AP assignsspace-time streams to individual stations, assigns SUT indexes tostations, and generates other system-wide or station-specific PHYparameters, according to an embodiment. The AP then transmits asynchronization frame to two or more stations to announce the schedulingof the SUT time period as well as the relevant parameters.

At block 604, the AP simultaneously receives several SUT communicationframes. As discussed above, the communication frames in some embodimentsinclude both legacy and VHT portions. In an embodiment, the AP estimatesthe MIMO channel between the AP and the virtual transmit antenna arraydefined by two or more SUT-capable stations using PHY preambles of thereceived communication frames. In at least some of the embodiments, theAP demultiplexes station-specific portions of communication frames inview of the previously defined space-time mapping. In some embodiments,the received communication frames include station-specific PHYparameters not assigned or controlled by the AP.

At block 606, the AP generates one or several acknowledgement frames. Inan embodiment, the one or several acknowledgement frames are transmittedat the end of the SUT period. In one embodiment, the AP generates andbroadcasts a single “block” acknowledgement frame that identifies eachstation from which a SUT communication frame has been successfullyreceived. In another embodiment, the AP generates and transmits aplurality of individual acknowledgement frames to each station fromwhich a SUT communication frame has been successfully received,

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. Likewise, the software or firmware instructionsmay be delivered to a user or a system via any known or desired deliverymethod including, for example, on a computer readable disk or othertransportable computer storage mechanism or via communication media.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism. The term“modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency,infrared and other wireless media. Thus, the software or firmwareinstructions may be delivered to a user or a system via a communicationchannel such as a telephone line, a DSL line, a cable television line, afiber optics line, a wireless communication channel, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium). The software orfirmware instructions may include machine readable instructions that,when executed by the processor, cause the processor to perform variousacts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe invention.

What is claimed is:
 1. A method for use in a communication network, themethod comprising: generating, at a first device, a synchronization dataunit to be transmitted to a plurality of second devices, wherein thesynchronization data unit (i) includes information for schedulingsubsequent simultaneous transmission of a plurality of data units by theplurality of second devices, and (ii) specifies a space-time mappingparameter to be utilized subsequently by the plurality of second deviceswhen transmitting the plurality of data units to the first communicationdevice; assigning a unique index to each of the plurality of seconddevices, so that each of the plurality of second devices maps at least aportion of a respective one of the plurality of data units to a set ofspace-time streams in accordance with the respective unique index; andsubsequently receiving, at the first device, the plurality of data unitsvia a plurality of antennas, wherein the plurality of data units aretransmitted simultaneously from respective ones of the plurality ofsecond devices in accordance with the space-time mapping parameter. 2.The method of claim 1, further comprising generating a respectiveacknowledgement data unit for each of the plurality of second devicesfrom which a data unit has been successfully received.
 3. The method ofclaim 1, further comprising generating an acknowledgement data unit thatidentifies each of the plurality of second devices from which a dataunit has been successfully received.
 4. The method of claim 1, whereinthe synchronization data unit specifies at least one of: a duration of atime period during which the plurality of second devices cansimultaneously transmit the plurality of data units; a maximum length ofa data unit to be transmitted simultaneously with others of theplurality of data units; and a time interval between the transmission ofthe synchronization data unit and the transmission of the plurality ofdata units.
 5. The method of claim 1, wherein the synchronization dataunit specifies a respective modulation and coding scheme (MCS) for eachof the plurality of second devices.
 6. The method of claim 1, whereinthe synchronization data unit specifies a respective amount of bandwidthallocated to each of the plurality of second devices.
 7. The method ofclaim 1, wherein the plurality of antennas and respective antennas ofthe plurality of second devices define a multiple-input, multiple output(MIMO) communication channel including a plurality of space-timestreams; and wherein the space-time mapping parameter specifies arespective set of space-time streams assigned to each of the pluralityof second devices, wherein each set of space-time streams is a uniquesub-set of the plurality of space-time streams.
 8. The method of claim7, wherein a matrix P specifies a mapping of symbols to the plurality ofspace-time streams of the MIMO channel; and wherein each of theplurality of second devices uses a set of rows of the matrix P totransmit at least a portion of the corresponding one of the plurality ofdata units in accordance with the assigned set of space-time streams. 9.The method of claim 7, wherein each of the plurality of data unitsincludes a training portion to be used in estimating the MIMO channeland another portion to convey information specific to the correspondingone of the plurality of second devices.
 10. The method of claim 1,further comprising: de-multiplexing the plurality of data units inaccordance with the unique index of each of the plurality of seconddevices.
 11. The method of claim 1, wherein receiving the plurality ofdata units includes receiving the respective beginning of each of theplurality of data units within a time period equal to the duration of acyclic prefix (CP) of an orthogonal frequency-division multiplexing(OFDM) symbol.
 12. The method of claim 1, further comprising schedulinga time period during which the plurality of second devices cansimultaneously transmit the plurality of data units; wherein thesynchronization data unit specifies a timing of the scheduled timeperiod; and wherein the plurality of data units are received during thescheduled time period.
 13. The method of claim 12, wherein the timeperiod is a first time period; the method further comprising: schedulinga second time period; and receiving data from a single legacy deviceduring the second time period, wherein the legacy device is notconfigured to transmit data to a receiver simultaneously with anotherdevice.
 14. A first communication device, comprising: a synchronizationcontroller configured to generate a synchronization data unit, whereinthe synchronization data unit (i) is to be transmitted to a plurality ofsecond communication devices, (ii) includes information for schedulingsubsequent simultaneous transmission of a plurality of communicationframes by the plurality of second communication devices, and (iii)specifies a space-time mapping parameter to be utilized subsequently bythe plurality of second communication devices when transmitting theplurality of communication frames to the first communication device, andassign a unique index to each of the plurality of second communicationdevices so that each of the plurality of second communication devicesmaps at least a portion of a respective one of the plurality ofcommunication frames to space-time streams in a MIMO communicationchannel in accordance with the respective unique index; and acommunication frame processor configured to process a signal receivedvia a plurality of antennas after the synchronization data unit istransmitted, wherein the received signal includes the plurality ofcommunication frames transmitted simultaneously from respective ones ofthe plurality of second communication devices in accordance with thespace-time mapping parameter.
 15. The first communication device ofclaim 14, wherein the communication frame processor includes ademultiplexer configured to demultiplex the received signal into theplurality of communication frames.
 16. The first communication device ofclaim 15, further comprising a P matrix controller configured to controla mapping of symbols to space-time streams in a MIMO communicationchannel between the apparatus and the plurality of second communicationdevices, wherein the mapping is common to the apparatus and theplurality of second communication devices, and wherein the plurality ofcommunication frames are transmitted in accordance with the mapping. 17.The first communication device of claim 14, further comprising anacknowledgement generator configured to generate at least oneacknowledgement frame to be transmitted to at least one of the pluralityof second communication devices.
 18. The first communication device ofclaim 14, wherein the synchronization controller is configured to assigna respective portion of a MIMO channel to each of the plurality ofsecond communication devices.
 19. The first communication device ofclaim 14, further comprising: a legacy portion processor configured toprocess a first portion of a physical layer (PHY) preamble of a receivedcommunication frame; and a very high throughput (VHT) portion processorconfigured to process a second portion of the PHY preamble of thereceived communication frame, wherein the first portion of the PHYpreamble is the same for each of the plurality of second communicationdevices, and the second portion of the PHY preamble is specific to eachof the plurality of second communication devices.
 20. A method for usein a communication network, the method comprising: generating, at afirst device, a synchronization data unit to be transmitted to aplurality of second devices, wherein the synchronization data unit (i)includes information for scheduling subsequent simultaneous transmissionof a plurality of data units by the plurality of second devices, and(ii) specifies a space-time mapping parameter to be utilizedsubsequently by the plurality of second devices when transmitting theplurality of data units to the first communication device; andsubsequently receiving, at the first device, the plurality of data unitsvia a plurality of antennas, including receiving the respectivebeginning of each of the plurality of data units within a time periodequal to the duration of a cyclic prefix (CP) of an orthogonalfrequency-division multiplexing (OFDM) symbol, the plurality of dataunits having been transmitted simultaneously from respective ones of theplurality of second devices in accordance with the space-time mappingparameter.
 21. A method for use in a communication network, the methodcomprising: scheduling a first time period during which the plurality ofsecond devices can simultaneously transmit the plurality of data units;generating, at a first device, a synchronization data unit to betransmitted to a plurality of second devices, wherein thesynchronization data unit (i) includes information for schedulingsubsequent simultaneous transmission of a plurality of data units by theplurality of second devices, the information for scheduling specifying atiming of the first time period, and (ii) specifies a space-time mappingparameter to be utilized subsequently by the plurality of second deviceswhen transmitting the plurality of data units to the first communicationdevice; and subsequently receiving, at the first device, the pluralityof data units via a plurality of antennas during the first time period,the plurality of data units having been transmitted simultaneously fromrespective ones of the plurality of second devices in accordance withthe space-time mapping parameter; scheduling a second time period; andreceiving data from a single legacy device during the second timeperiod, wherein the legacy device is not configured to transmit data toa receiver simultaneously with another device.